The bar at the very bottom of the screen has gone away?
Click on a transparent '+' button in the down right part of a screen.


Maybe you accidentaly collapsed menus. Hover your cursor over the bottom bar, then press RMB and uncheck the Collapse Menus box.


If you want to have many different windows (editors) opened at once you can add them by:


clicking on the corner of the window and dragging it in any diection:

using a Split Area option (press the RMB while hovering cursor over the border of the window)



With multiple windows opened you can change the editor type for each of them by clicking on area type button as I've shown above. In your case change the editor type for one of them to Timeline.